Redfield ratio

Redfield ratio or Redfield stoichiometry is the molecular ratio of carbon, nitrogen and phosphorus in plankton. This empirically developed stoichiometric ratio is found to be C:N:P = 106:16:1. This term is named after the American oceanographer Alfred C. Redfield, who first described this ratio in an article written in 1934 (Redfield 1934). As a physiologist, Redfield participated in several voyages on board the research vessel Atlantis. Alfred Redfield analyzed thousands of samples of marine biomass across all of the ocean regions. From this research he found that globally the elemental composition of marine organic matter (dead and living) was remarkably constant across all of the regions. The stoichiometric ratios of carbon, nitrogen, phosphorus remain relatively consistent from both the coastal to open ocean regions.

History

In his 1934 paper, Alfred Redfield proposed that the ratio of Nitrogen to Phosphorus in plankton resulted in the global ocean having a remarkably similar ratio of dissolved nitrate to phosphate (16:1). Redfield felt that it wasn’t purely a coincidence that the large oceans would have a chemistry perfectly suited for the requirements of life.

In his hypothesis he suggested that if the ocean were to be devoid of life then the chemical compositions would be significantly different from its actual composition. He determined this ratio empirically by analyzing thousands of samples of marine biomass from all of the ocean regions.

In 1958, almost a quarter century after first discovering the ratios, Redfield proposed the seminal idea of "the biological control of chemical factors" in the ocean (Redfield, 1958). He considered how the cycles of not just N and P but also C and O could interact to result in this match.

Uses

This research has resulted in this ratio has become a fundamental feature in the understanding of the biogeochemical cycles of the oceans. They also help in determining which nutrients are limiting in a localized system, if there is a limiting nutrient. The ratio can also be used to understand the formation of phytoplankton blooms and subsequently hypoxia by use of comparison of the ratio between different regions, such as a comparison of the Redfield Ratio of the Mississippi River to the ratio of the northern Gulf of Mexico.

Explanation

In the ocean a large portion of the biomass is found to be nitrogen-rich plankton. Many of these plankton are consumed by other plankton biomass which have similar chemical compositions. This results in a similar nitrogen to phosphorus ratio, on average, for all the plankton throughout the world’s ocean, averaging approximately 16:1. When these organisms sink into the ocean interior, their energy-rich bodies are consumed by bacteria that, in aerobic conditions, oxidize the organic matter to form dissolved inorganic nutrients, mainly carbon dioxide, nitrate, and phosphate.

Redfield explained the remarkable congruence between the chemistry of the deep ocean and the chemistry of living things such as phytoplankton in the surface ocean. Both have N:P ratios of about 16:1 in terms of atoms. When nutrients are not limiting, the molar elemental ratio C:N:P in most phytoplankton is 106:16:1. Redfield thought it wasn't purely coincidental that the vast oceans would have a chemistry perfectly suited to the requirements of living organisms.

Although the Redfield ratio is remarkably stable in the deep ocean, phytoplankton may have large variations in the C:N:P composition, and their life strategy play a role in the C:N:P ratio, which has made some researchers speculate that the Redfield ratio perhaps is a general average rather than specific requirement for phytoplankton growth (e.g., Arrigo 2005) as no theoretical justification for Redfield ratio has ever been found.

Errors with the Redfield Ratio

This ratio was initially derived empirically from measurements of the elemental composition of plankton in addition to the nitrate and phosphate content of seawater collected from a few stations in the Atlantic Ocean. This was later supported by hundreds of independent measurements. However, looking at the composition of individual species of phytoplankton grown with nitrogen and phosphorus limitation shows that this nitrogen to phosphorus ratio can vary anywhere from 6:1 to 60:1. While understanding this problem, Redfield never attempted to explain it with the exception of noting that the N:P ratio of inorganic nutrients in the ocean interior was an average with small scale variability to be expected.

Despite reports that the elemental composition of organisms such as marine phytoplankton in an oceanic region do not conform to the Redfield ratio, the fundamental concept of this ratio continues to remain valid. That the nitrate to phosphate ratio in the interior of all of the major ocean basins is highly similar to the N:P ratio is due to the residence times of these elements in the ocean relative to the oceans circulation time, roughly 10,000 years and 1000 years, respectively. The fact that the residence times of these elements are greater than the mixing times by an order of magnitude results in the ratio of nitrate to phosphate in the ocean interior remaining fairly constant.

Modified Redfield Ratio

Some feel that there are other elements, such as Potassium, Sulfur, Zinc, Copper, and Iron are also important in the ocean chemistry. In particular, iron (Fe) was considered of great importance as early biological oceanographers hypothesized that iron may also be a limiting factor for primary production in the ocean. As a result a Modified Redfield Ratio was developed to include this as part of this balance. This new stoichiometric ratio states that the ratio should be 106 C:16 N:1 P:0.1-0.001 Fe. The variation in iron is the result of “…iron contamination on ships and in labs is large and difficult to control. No one has been able to beat this nearly insuperable combination of difficulties.” (Broecker and Peng (1982)). It is this contamination that resulted in early evidence suggesting that iron concentrations were high and not a limiting factor in marine primary production.

Redfield Ratio in Diatoms

Diatoms need, among other nutrients, silicic acid to create biogenic silica for their frustules (cell walls). As a result of this the Redfield-Brzezinski nutrient ratio was proposed for diatoms and stated to be C:Si:N:P = 106:15:16:1 (Brzezinski, 1985).

See also

• ecological stoichiometry

References

• Arrigo, K.R., Marine microorganisms and global nutrient cycles, Nature, Vol 437, pp. 349-355, 2005
• Brzezinski, M.A., The Si:C:N ratio of marine diatoms: interspecific variability and the effect of some environmental variables. Journal of Phycology, Vo. 21, pp. 347-357, 1985
• Johnson, Zackary. "Biogeochemistry IV." University of Hawaii School of Ocean and Earth Science and Technology. Web. <http://www.soest.hawaii.edu/oceanography/zij/ocn621/OCN621-20060215-biogeochemistry.pdf>.
• Lentz, Jennifer. "Nutrient Stoichiometry - Redfield Ratios." LSU School of the Coast and Environment, 2010. Web. <http://www.sce.lsu.edu/cego/Documents/Reviews/Oceanography/Nutrient_Stoichiometry.pdf>.
• P.G. Falkowski, and C.S. Davis. "MARINE BIOGEOCHEMISTRY: ON REDFIELD RATIOS." ScienceWeek. Nature, 2004. Web. <http://scienceweek.com/2004/sa041119-5.htm>.
• Redfield A.C., On the proportions of organic derivations in sea water and their relation to the composition of plankton. In James Johnstone Memorial Volume. (ed. R.J. Daniel). University Press of Liverpool, pp. 177-192, 1934.
• Redfield, A.C., The biological control of chemical factors in the environment, American Scientist, 1958